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Threads are everywhere—but designing them right for CNC machining? That’s where most mistakes happen.
We’ve seen thread failures caused by too-shallow holes, mismatched pitches, or blind holes with nowhere for the tap to escape. And while CAD makes it easy to drop a threaded hole feature, CNC threads design requires a much deeper understanding of both geometry and process.
Whether you’re designing internal tapped holes or external threads on shafts, what works on screen may not translate well to the machine shop—or worse, to the final assembly.
Internal vs External Threads: Design Decisions That Matter
Internal vs External Threads: Design Decisions That Matter
In CNC threads design, internal threads are created by tapping, thread milling, or using forming tools inside predrilled holes. External threads are usually turned on lathes using profile tools or thread turning cycles.
Internal Threads (Tapped Holes)
Leave at least 1.5× pitch clearance below threads
Through-holes improve chip evacuation
60–70% thread engagement is usually strong enough
External Threads (Turned Threads)
Use relief grooves to avoid burrs
Add unthreaded shank for inspection
Check lathe’s thread pitch capabilities
Common Thread Standards & What They Actually Mean
Common Thread Standards & What They Actually Mean
Metric (ISO)
Default for global designs (e.g., M6×1)
Use tolerance class 6H (internal) / 6g (external)
Unified (UNC/UNF)
Standard in U.S. and aerospace
UNF = finer pitch, better in thin-walled parts
Special Threads (Acme, Trapezoidal, Buttress)
Used in linear motion and heavy loads
Confirm with shop if special tools are needed
Tapped Hole Design Pitfalls: Blind Holes, Bottoming Taps & Chip Clearance
Tapped Hole Design Pitfalls: Blind Holes, Bottoming Taps & Chip Clearance
Blind holes require an extra 1.5–2× pitch of clearance for the tap to run out. A conical relief at the bottom can reduce tool breakage and support full-thread formation.
For aluminum or blind holes, use chip-lifting taps or consider thread milling.
Threads in Soft Materials: Should You Tap or Use Inserts?
Threads in Soft Materials: Should You Tap or Use Inserts?
Helicoil Inserts
Ideal for aluminum and magnesium
Reinforced threads for repeated use
Thread Forming
Material is displaced, not cut
Higher fatigue resistance, no chips
Brass Inserts for Plastics
Heat-set into ABS, Delrin, nylon
Avoid direct tapping in soft polymers
FAQ: CNC Threads Design Questions We Hear All the Time
FAQ: CNC Threads Design Questions We Hear All the Time
Q: How deep should I tap a blind hole?
A: Add 1.5–2× thread pitch below the thread to ensure tool runout and proper chip evacuation.
Q: Are Helicoils better than cutting threads in aluminum?
A: Yes. Especially when the part will be assembled repeatedly or sees torque loads.
Q: Can I use metric and imperial threads in one part?
A: You can—but check your CAM settings, drill sizes, and tap availability carefully.
Q: Can threads be 3D printed instead of machined?
A: For cosmetic parts only. For real performance, machined threads are far more reliable.
Related Standards & Technical References
Related Standards & Technical References
ISO 68-1:1998 – Metric Thread Profile
ASME B1.1 – Unified Inch Screw Threads
Emuge Tapping Manual: www.emuge.com
Heli-Coil System Guide: www.stanleyengineeredfastening.com
How-To: Design Threaded Holes for CNC
Watch: CNC Threading Basics (Video)
Watch: CNC Threading Basics (Video)
Want a 3-minute visual explanation of CNC threads, blind hole tips, and insert options?
Watch on YouTube
Need Custom Threaded Parts?
Need Custom Threaded Parts?
Upload your part drawing to get real-time DFM feedback, optimized tap depth suggestions, or insert recommendations.
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